Digital representation, storage, distribution, and duplication of digital media have become very popular because they are inexpensive, easy to use, and maintain the quality of the media. These advantages however have enabled widespread, illegal distribution and use of copyrighted material, such as unauthorized distribution of digital images and videos over the Internet.
Many different approaches have been presented to secure digital media against unauthorized use. For example, digital encryption technology is effective to enable secure communication and delivery. However, if encrypted content is decrypted or presented in a form that is visible or audible to humans, the content can be re-recorded and an unsecured copy can be obtained.
Marking media by embedding recipient information in the media can help identify individuals that receive the media and use the content in an unauthorized manner. Further, embedded ownership information in the media can indicate copy restriction and clarify ownership of the media.
One way of marking media is by adding annotations to the digital media file format that can be read from the unmodified file and that are ignored during playback. This information can be lost, however, if the file is re-recorded or converted to another format.
To achieve a robust and permanent mark in video media, visible and overlay images that display copyright information during playback have been proposed. Overlaid images are robust against modification and easy to read. However, this approach can reduce the quality of the marked video, because the overlaid images interfere with the original video, which diminishes the quality of the viewing experience. In addition, overlaid images are obvious and are therefore easy to identify, and can be removed by overwriting or cropping.
Digital watermarking is another approach that has been suggested in several different variations. Many common digital watermarking schemes involve embedding a digital bit sequence in a digital media by introducing machine readable manipulations at certain positions in space or time. During readout, software is used to interpret these manipulations and to derive a digital bit sequence that is used to assemble the embedded message. In order to interpret the manipulations, knowledge of the positions of the manipulations in space or time is required. When the manipulations are distorted (i.e., misplaced or weakened, for example), the readout (also referred to as detection) often becomes difficult or impossible. Distortions can occur during simple media treatment such as cropping, rotation, conversion to another file format, and modification of the frame rate. Further, intentional misplacement of the manipulations can be applied by an attacker in order to remove the mark, and can be achieved through imperceptible, slight, combined distortions, such as shifts, rotation, and variations in playback speed. Publicly available tools apply some of these manipulations, also called attacks, in an automated fashion. Current digital watermarking algorithms are often not powerful enough to recognize misplacements in distorted content (a process also called registration). As a result, intentional misplacement can render the digital watermark unreadable.
Machine readable manipulations are also vulnerable to another security risk, which is described below. Detection of machine readable manipulations typically requires knowledge of the manipulations that have been performed. Therefore, someone attempting to circumvent a watermark can ascertain location and meaning of the manipulations by observing the watermark being read or embedded. The observation can be performed by analyzing, or reverse engineering, the embedding or detection process. Subsequently, the manipulations can be removed or inverted by a skilled attacker, effectively removing the watermark. In addition to removal of the watermark, the reverse engineering approach described above enables modification of the information embedded in the watermark. This is true, even if the positions of the manipulations are encrypted with a secret key. The protection offered by the use of a secrete key is limited, because the same key is typically used for embedding and detection. An attacker can analyze or reverse engineer the embedding or detection application and gain access to the locations, even if they are encrypted. Furthermore the secret key can be observed by analyzing the detection or embedding application.
In addition to registration and security, robustness against lossy compression and filtering is an important component of imperceptible marking of multi-media content. During watermarking, a message is typically embedded in digital media by manipulating areas of the digital media that are suited for hiding machine-readable information and do not significantly contribute to the human perception of the marked content. Examples of such areas in the digital media include areas that include fine details (i.e., high frequencies). These areas however, can be altered or removed while the content maintains an acceptable quality. For example, common lossy compression schemes like MPEG2 and H.264 remove this perceptually insignificant information in order to reduce the size of a digital media file and thereby remove watermark information stored there. Therefore, compression of the media using such a lossy compression scheme can result in the removal of some or all of the watermark information.
Digital still images have been the early focus of watermarking research. Video watermark approaches are typically based on the application of a still image watermark to each video frame. The reason is that the application of the still image watermark to each video frame is obvious and easy to implement. However, this approach does not efficiently use the time domain for gathering embedded information. Detection of the watermark is typically only successful if some information from the individual frames can be recovered. This approach often fails if the watermark cannot be read in any frame due to a failure in registration or destruction of relevant areas of the video frames. Furthermore, if frames are watermarked as individual images and the watermarks vary between frames, the watermarks are susceptible to an attack because similar frames within one video can be averaged together in order to weaken the watermark. If each of the frames contains an identical watermark, the frames can be used to analyze the structure of the watermark that is in each of the frames to understand and subsequently remove the watermark.
Furthermore, the process of digital watermarking typically involves a complex transformation of the original image and of the message to be embedded. Examples of such complex transformations are DCT, Fast Fourier, or Wavelet transformations. The required calculations to perform these transformations are time intensive processes, which can be a significant limitation when embedding a digital watermark in real-time (e.g., during playback or download). Furthermore, watermarking approaches typically consist of clearly defined modifications, allowing little flexibility for variations in order to adopt different compression formats, security, or performance requirements.